Various emerging food-processing technologies, notably ultrasonics, high-pressure processing and microwave technology are increasingly being explored and used in oil and phytonutrient recovery and in food processing operations. There is potential for the application of emerging food processing technologies in the palm oil industry. The potential applications in palm milling operations include: micro-wave assisted extraction of palm oil, ultrasound-assisted extraction and recovery of palm oil and palm phytonutrients and high pressure processing for oil and emulsion products. As the palm oil industry moves into the future, there will also be a need for improving the sustainability of palm oil processing and to reduce the carbon footprint and energy requirements of the overall processes.
Traditionally ultrasound technology has been presented as a potential processing intervention for assisting extraction, microbial inactivation, emulsification or homogenisation and de-emulsification. Ultrasonic-assisted extraction of different vegetable materials has been studied. For example, ultrasound can assist in the extraction of bioactive components from herbs at laboratory and large scale. The mechanism of ultrasonic extraction is based on the effect of sonication in breaking vegetal cells and improving diffusion and capillary processes. Ultrasound, through mechanical effects, disrupts plant cells. This facilitates penetration of an extractant into the plant cell mass, enhancing mass transfer (Mason T J, Paniwnjk L, Lorimer J P. The uses of ultrasound in food technology. Ultrasonics Sonochem 1996; 3:S253-26).
This may result in an increase in the extraction efficiency as well as extraction rate.
In addition, ultrasound has an effect on increasing the swelling of vegetal tissue, facilitating cell wall rupture and releasing intracellular components into water during sonication. Extraction enhancement by ultrasound has been attributed to the propagation of ultrasound pressure waves and resulting cavitation phenomena.
Ultrasonic separation of food ingredients has obvious advantages over conventional methods like filtration and natural settling.
Most examples to date involve the use of ultrasound in combination with organic solvents for improving the extraction of oil and oil soluble components. Examples include extraction of soy oil (Li H, Pordesima L, Weiss J. High intensity ultrasound-assisted extraction of oil from soybeans. Food Res Int 2004; 37:731-738), and soy isoflavone (Rostango M A, Palma M, Barroso C G. Ultrasound-assisted extraction of soy isoflavones. J Chromatog A 2003; 1012:119-128).
GB patent 2097014 discloses a solvent extraction method using hexane in which ultrasonic agitation at from 20 to 60 kHz for residence times of 5 to 30 seconds. European patent 243220 discloses a similar process for extracting oil from seeds using ultrasonic frequencies of 10 to 50 kHz on the seeds suspended in a solvent.
Methods also exist for ultrasound-assisted extraction in the absence of organic solvents. WO2010138254 discloses the use of ultrasound-assisted extraction of oil from palm oil mill effluent, where the aim is to increase oil recovery and reduce biochemical and chemical oxygen of palm oil mill effluent.
Ultrasonic separation of food ingredients has obvious advantages over conventional methods like filtration and natural settling. While the above background relates to extraction the subject presented here relates to separation post digestion and or maceration. The method adopted is based on the principle of standing wave fields.
In this invention we seek to employ standing waves at high ultrasonic frequencies typically greater than 400 kHz to facilitate the separation of oil from vegetal solids. It is a limitation of current ultrasonic equipment design and material limitations that at frequencies above 100 kHz it is not feasible to use any form of ultrasonic horn to propagate ultrasound. Current ultrasonic horn designs generally enable operation between 20 to 24 kHz. This means that, unlike the piezo-electric wafer stacks used to drive horn transducers, single wafer piezo-electric transducers bonded to plate surfaces are required to achieve frequencies above 100 kHz. Plate transducers operate at specific amplitudes very much lower than those accomplished by horn transducers.
At frequencies greater than 400 kHz it is practical to produce large area standing waves at low amplitudes. Pangu & Feke, 2007 and Nii et al., 2009, disclose that standing waves accomplish phase separations on the basis of the relative specific gravities of the phases. So when oil is dispersed in water primary acoustic forces will separate the oil to the wave antinodes. In the work which is the subject of these citations biphasic oil and water systems are studied at an ultrasonic frequency of 2 MHz. Further these studies teach that in order to obtain coalescence of the oil it is necessary for secondary acoustic forces perpendicular to the standing wave plane to develop as a result of the wave field being bounded by walls perpendicular to the plane of the waves. The minimum temperature at which standing waves can be used to separate oil from water is limited by the increasing viscosity of the oil as the temperature is reduced. Ideally for triglyceride vegetable oils the temperature should be as low as is practical to minimize the potential for hydrolysis of free fatty acids, oxidation of unsaturated fatty acids and destruction of sensitive phytochemicals inherent in the oils. The present invention can address the issue of reducing the temperature of current water based vegetable oil separation processes and therefore yield an increased quality.
In the situation where separation of oil from vegetal material is suspended in water a tri-phase system exists. In such a system the oil has a lower specific gravity than the other phases and will migrate to the antinodes and the residual vegetal material having a higher specific gravity than water will migrate to the nodes. In this situation the relative radius of the vegetal particles compared to the half wave length of the standing wave must be smaller; otherwise complete separation of the oil from the vegetal material will not occur. Reducing the standing wave frequency will increase the wave length and enable oil to be separated from larger vegetal particles, however the separation time is lengthened and it becomes more difficult to maintain a stable standing wave field. In a situation where the treatment is carried out at reduced temperatures enzymes such as cellulase and polygalacturonase can be introduced into the system to facilitate non-mechanical breakdown of the vegetal material (Priego-Capote & Luque de Castro, 2007), which in turn will enable the application of higher standing wave frequencies.
The conventional method for extraction of palm oil is to use a press such as a screw press to extract an oil containing liquid and then allow the oil to separate out and recover the oil. Currently the process in the extraction and recovery of palm oil involves (a) sterilisation of the fresh fruit bunches, (b) stripping the fruit from the bunch by mechanical means, (c) steeping the fruit in hot water followed by mechanical expression of the oil, typically using a screw-press, (d) settling the oil-water-residual solids mixture in a settling tank. The oil that rises to the top of the settling tank is drawn off, clarified and dried. The sludge (i.e. underflow from settling tank) is centrifuged to recover further oil which is returned to the settling tank. The sludge (ex-centrifuge) also some contains residue oil (Berger K, Production of palm oil from fruit. JAOCS 60(2), 206-210, 1983). The process is depicted in FIG. 1.
The economics of the palm oil extraction are such that a 1% increase in oil yield is economically significant.
It is desirable to improve the yield of the oil extraction process.